The invention relates generally to noise reduction systems. In particular, the invention relates to negative output resistance amplifiers in noise reduction systems, and more particularly to temperature-compensated negative output resistance amplifiers in noise reduction systems.
Automatic Noise Reduction (ANR) systems cancel or reduce unwanted acoustic waves by generating an out-of-phase response, thereby canceling out the unwanted waves. FIG. 1 depicts an ANR system 100 having a microphone 110, a filter 120 and a speaker 130.
In referring to FIG. 1, the combination of the microphone 110, filter 120 and speaker 130 form a transfer function G(f)=Output(f)/Input(f). This creates a closed-loop control system that reduces ambient noise around the microphone according to the function 1/(1xe2x88x92G(f)).
ANR can be used in a variety of applications. For example, an ANR system may be placed near the muffler of a motor vehicle to reduce vehicle noise emissions. Also, an ANR system can be incorporated in a headset. Such an ANR headset can be worn by construction workers to protect their hearing. Similarly, the ANR headset can be worn by airplane pilots whose ability to hear may suffer from engine noise.
FIG. 2 illustrates one embodiment of an ANR headset 200 worn by a user 201. The ANR headset 200 includes two cups 240, each of which fits over an ear 202 of the user 201. Each cup 240 is enclosed by a cup wall 235. The cup 240 is sealed about the ear 202 by a cushion 205 to diminish undesired noise from reaching the user""s ear 202, and to provide the user 201 with a comfortable fit.
The cup 240 also includes a speaker 220. The speaker 220 broadcasts the out-of-phase audio signal. The speaker 220 also defines front and rear cavities, 245 and 250 respectively, in the cup 240.
A microphone 210 is inserted in the front cavity 245 proximate to the user""s ear 202. The microphone 210 receives the audible noise. The microphone 210 is coupled through a filter 225 to the speaker 220. Optionally, for ANR headsets 200 worn by users that must receive audio communication signals, a signal summer 215 is inserted between the microphone 210 and filter 225. The signal summer 215 is connected to an audio output 230 that permits the user 201 to listen to desired audio signals while reducing undesired ambient noise. For example, this technique permits an airplane pilot to listen to radio communications even when ambient noise is being suppressed by the ANR system. The filter 225 and summer 215 can be incorporated in the ANR headset 200, such as in the cups 240, or they may be positioned externally with respect to the cups 240.
The electroacoustic combination of each cup""s speaker, and front and rear cavities create relatively high Q resonances in the audio frequency response of the speaker. The resonances"" amplitudes and frequencies can readily change as a result of variations in cup and speaker construction. Further, the resonances"" amplitudes and frequencies can also readily change as a result of variations in cavity dimensions which may result from varying headset positions on different users, and varying shapes of users"" heads and ears.
A speaker 220, and its resonances, can be modeled by a lumped equivalent circuit, as illustrated in FIG. 3. RE represents the resistance of the wire coil of the speaker. A represents the area of the speaker""s diaphragm. MM represents the moving mass of the speaker. RM represents the speaker""s mechanical damping associated with suspension of the wire coil. CM represents the speaker""s compliance associated with suspension of the diaphragm. ZC is the acoustic impedance that terminates the speaker""s diaphragm. Finally, ZLOAD is the input impedance seen across the speaker input terminals.
To permit relatively uniform ANR across the audible frequency range, the high Q response of the speaker is equalized, or diminished. To this end, an equalization filter is included in the filter 225 of the ANR system, described above. The equalization filter typically must cancel complex pole-zero pairs because of the cup""s high Q frequency response. Because of the cup""s high Q frequency response, the equalization is sensitive to, and can be diminished by, minor variations in operating parameters, such as headset fit on a user and component variations. To diminish the relatively high Q response of the cup, fabric is often placed over vents in the back of the speakers. The fabric dampens the frequency response of the speakers, thus reducing the Qs of the resonances. However, as a result, the fabric also undesirably diminishes the efficiency of the speakers, and provides variable changes in performance.
Further, such an equalization filter is relatively costly because of the number of required parts necessary to cancel the complex pole-zero pairs. One embodiment of an ANR filter 225 incorporating an equalization filter 410 and a noise reduction filter 420 is illustrated in FIG. 4.
The ANR filter 225 provides the correct open-loop response for G(f) so that the closed-loop response of the ANR headset 200 provides high gain (i.e., high noise cancelation) and closed-loop stability.
It has been proposed by St{dot over (a)}hl in U.S. Pat. No. 4,118,600, issued Oct. 3, 1978, that the bass response of a loudspeaker can be improved by including a negative impedance in series with a plurality of impedances connected in parallel, such that the negative impedance (including negative resistance) is chosen to be substantially equal to the impedance of the voice-coil of the loudspeaker. St{dot over (a)}hl proposed that the plurality of parallel impedances have values which cause the loudspeaker to exhibit apparent mechanical parameters which are substantially different from the actual mechanical parameters in the bass response of the loudspeaker.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for ANR systems capable of diminishing the Q of the frequency response of the speaker, without reducing speaker efficiency.
The present invention provides a method of enhancing automatic noise reduction in a headset speaker using a negative output resistance to substantially eliminate the coil resistance of the speaker. In one embodiment, the method includes generating a negative output resistance substantially equal in magnitude to the coil resistance of the speaker, and serially combining the negative output resistance with the coil resistance of the speaker. In another embodiment, generating a negative output resistance includes generating a negative output resistance using a negative output resistance amplifier. In a further embodiment, generating a negative output resistance includes generating a negative output resistance using a single-ended negative output resistance amplifier. In yet another embodiment, generating a negative output resistance includes generating a negative output resistance using a balanced negative output resistance amplifier.
The invention further provides a method of temperature compensating a system having a negative output resistance amplifier and a resistive load. In one embodiment, the method includes coupling a negative output resistance amplifier to the resistive load, and temperature compensating the negative output resistance amplifier so that a temperature coefficient of the negative output resistance is approximately equivalent to a temperature coefficient of the resistive load. In another embodiment, temperature compensating the negative output resistance amplifier includes implementing a resistor in the negative output resistance amplifier having a temperature coefficient substantially equivalent to the temperature coefficient of the resistive load, wherein the output resistance of the negative output resistance amplifier is directly proportional to the resistance of the resistor and wherein remaining resistors have resistances which are substantially temperature invariant. In a further embodiment, temperature compensating the negative output resistance amplifier includes implementing a resistor in the negative output resistance amplifier having a temperature coefficient substantially equivalent to the inverse of the temperature coefficient of the resistive load, wherein the output resistance of the negative output resistance amplifier is inversely proportional to the resistance of the resistor and wherein remaining resistors have resistances which are substantially temperature invariant. In a still further embodiment, temperature compensating the negative output resistance amplifier includes implementing at least two resistors in the negative output resistance amplifier having temperature coefficients such that their combination results in a temperature coefficient of the negative output resistance amplifier which is substantially equivalent to the temperature coefficient of the resistive load.
Another embodiment of the invention provides a method of diminishing the Q of the frequency response of a headset speaker using a temperature-compensated negative output resistance to substantially eliminate the coil resistance of the speaker. In one embodiment, the method includes generating a negative output resistance substantially equal in magnitude to the coil resistance of the speaker, temperature compensating the negative output resistance to substantially match the temperature variation of the coil resistance of the speaker, and serially combining the negative output resistance with the coil resistance of the speaker.
A further embodiment of the invention provides an automatic noise reduction headset. The automatic noise reduction headset includes a pair of cups, wherein each cup includes a speaker having a wire coil. The headset further includes a negative output resistance amplifier, having a negative output resistance, operatively coupled to each speaker to enhance automatic noise reduction. The headset further includes a filter operatively coupled to each negative output resistance amplifier and a microphone, in each cup, operatively coupled to each filter. In one embodiment, each negative output resistance amplifier is temperature compensated so that a temperature coefficient of the negative output resistance is approximately equivalent to a temperature coefficient of a resistance of the wire coil.
A further embodiment of the invention provides an automatic noise reduction headset having a negative output resistance amplifier coupled in series with the coil resistance of a headset speaker. In a still further embodiment, the negative output resistance amplifier is temperature compensated to substantially match the temperature variation of the coil resistance of the headset speaker.
Further embodiments of the invention include automatic noise reduction headsets produced in accordance with one or more methods of the invention. Such headsets are capable of diminishing the Q of the frequency response of the headset speakers in the headset cups, without adversely affecting speaker efficiency.